ClassCastException in HeaderCell.addToTotal when totaling column

I am getting a ClassCastException in org.displaytag.model.HeaderCell.addToTotal when value for cell is the empty string "". This is a problem when totaling a column using the total="true" attribute on ColumnTag.

This method should be changed to avoid this problem. Here is my proposed:

Original:
=====
    private void addToTotal(Object value)
    {
        if (value != null)
        {
            this.total = this.total + ((Number) value).doubleValue();
        }
    }
=====

Proposal:
=====
    private void addToTotal(Object value)
    {
        if (value != null && value instanceof Number)
        {
            this.total = this.total + ((Number) value).doubleValue();
        }
    }
=====